We report on the unexpected deterioration under ambient conditions of films of Co clusters capped with relatively thick (>100 nm) Cu (or Ti) layers deposited by either thermal evaporation or by radiofrequency sputtering. The magnetic character of the clusters, prepared by gas-phase condensation, allows monitoring the oxidation of the samples through the decay of the saturation magnetization, which takes place on a timescale of days. By contrast, diluted (<10 at.%) cluster-assembled granular Co:Cu films, prepared by co-deposition of the Co clusters with a Cu vapour, are perfectly stable under ambient conditions. We tentatively explain the oxidation of the cluster films as stemming from their very high porosity.